sedimentary
Ironstone Concretion
Ferruginous Sandstone / Siderite Concretion
Hardness: 4-5.5 (Mohs scale); Color: Rusty orange, reddish-brown, and dark gray; Luster: Earthy, dull, or metallic (near iron-rich parts); Structure: Clastic or massive; Specific Gravity: 3.5-4.5
- Hardness
- 4-5
- Color
- Rusty orange, reddish-brown, and dark gray
- Luster
- Earthy, dull, or metallic (near iron-rich parts)
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Physical properties
Hardness: 4-5.5 (Mohs scale); Color: Rusty orange, reddish-brown, and dark gray; Luster: Earthy, dull, or metallic (near iron-rich parts); Structure: Clastic or massive; Specific Gravity: 3.5-4.5
Formation & geological history
Formed by the precipitation of mineral cement, specifically iron oxides like limonite or hematite, within the spaces between sediment grains (sand or silt). These typically form in sedimentary basins or soil layers as groundwater rich in iron moves through the rock.
Uses & applications
Informational/Educational specimen, historical source of low-grade iron ore, and occasionally used as decorative garden stones.
Geological facts
These stones are often mistaken for meteorites because of their heavy weight and dark, weathered outer crust (desert varnish or iron oxide crust). They can sometimes form around a central nucleus, such as a fossil.
Field identification & locations
Identify by its high density compared to normal sandstone, rusty streak, and typical orange-red weathering. Common in arid environments or areas with high iron content in groundwater.
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